1. Field of the Invention
The present invention relates generally to a photographing method and apparatus, and more particularly to a photographing method and apparatus for photographing a fluorescent image of the fluorescent light emitted by a subject and a reflected light-image, etc. of a subject illuminated by a faint light.
2. Description of the Related Art
There has been expressed a desire for photographing a fluorescent image of the fluorescent light emitted by a subject and a reflected-light image of a subject illuminated by a faint light as clear images, and much discussion has ensued in regard to development of a high-sensitivity photographing system therefor.
For example, research has been conducted on an apparatus which, by analyzing fluorescent light-images obtained by photographing the fluorescent light emitted by the inherent dye of the structures of a living tissue illuminated by a stimulating light, facilitates the distinguishing of changes, etc. in the state of each type of disease. In a fluorescent endoscope, the image of the fluorescent light emitted by a subject is propagated along an image fiber and guided to the end thereof. The size of the image guided into the image fiber is magnified to a larger size than that at which it entered the image fiber, focused on a photographing element and photographed.
The fluorescent light emitted by the structures of a living tissue is faint, and a high-sensitivity photographing element is used to detect this light as an image. When the quantity of light received by the photographing element is large, the fluorescent image is photographed at the resolution corresponding to the number of pixels provided on the photographing element, however, when the quantity of light received by the photographing element is small, the signal charge of a plurality of pixels is multiplied, readout, subjected to a pixel binning processing, and the fluorescent image is photographed. For example, if an image fiber of a 2 mm diameter is constituted of 10,000 strands and the image formed by fluorescent light at the end of this fiber is formed of 10,000 pixels, the light-receiving zone of the photographing element for receiving this image is provided with 4 times the number of pixels (40,000 pixels or more). When, for example, the phosphor image received uses 25,000 or 38,000, etc. substantially all of the light-receiving zone of the photographing element, and when the quantity of fluorescent light received is small, the number of pixels to be subjected to pixel binning processing; that is, the number of pixels corresponding to 1 pixel is multiplied and increased, the quantity of light received corresponding to one pixel is increased and the resolution decreased, and conversely, when the quantity of fluorescent light received is large, the number of pixels to be subjected to pixel binning processing is reduced, the resolution increased and the image photographed.
More specifically, a desire has been expressed to obtain an image of a cancerous portion, which is located 50 mm away from the point on which the stimulated light is emitted, as an image signal having a S/N ratio of 1 or higher under standard photographing conditions, in which the structures of a living tissue are illuminated by a 120° wide, 100 mw stimulating light and exposed for 1/30 of a second over the entire light-receiving zone.
When photographing is performed using a front-exposure type photographing element under aforementioned standard photographing conditions, because the normal charge of the image signal representing the fluorescent image of aforementioned cancerous portion stored on 1 pixel of the light-receiving zone is substantially 10 electrons, by controlling the sum of the number of electrons of the readout noise of each pixel of the light-receiving zone and the number of electrons of dark noise to 10 electrons or less, a S/N ratio of 1 or higher can be obtained for the image signal representing the fluorescent light-image of the cancerous portion. In addition, for cases in which a rear-exposure type photographing element, which has a quantum efficiency substantially twice that of aforementioned front-exposure photographing element, is used, because the charge of the image signal representing the fluorescent image stored on 1 pixel under the settings of aforementioned standard photographing conditions is substantially 20 electrons, by controlling the sum of the number of electrons of the readout noise of each pixel of the light-receiving zone and the number of electrons of dark noise to 20 electrons or less, a S/N ratio of 1 or higher can be obtained for the image signal representing the fluorescent light-image of the cancerous tissue.
However, even if the above described method of employing a processing such as pixel binning is used, when the signal charge that has been stored on a plurality of pixels receiving the fluorescent light the fluorescent image is multiplied within the photographing element, because the signal charge generated due to reception of fluorescent light is in the end simultaneously multiplied together with the signal charge stored due to the dark noise, even if the charge of the image signal stored on a plurality of pixels is grouped together as a unit by being subjected to pixel binning processing and are and treated as corresponding to 1 pixel, the ratio of the dark noise component is not reduced, and an improvement in the S/N ratio cannot be hoped for. At this point, a photographing system in which the S/N ratio can be improved by reducing the dark noise component is desired.